Work has continued on the in vitro properties of activated iridium recording and stimulating microelectrodes. Parylene-C insulated microelectrodes have had their tips exposed with a computer controlled eximer laser. From scanning electron micrographs, before and after long term soaks in phosphate-buffered saline, the Parylene-C appears to remain tightly bond to the iridium at the exposure junction. The electrical properties of the activated iridium microelectrodes are a function of electrode potential. With a negative bias, the microelectrode is in a high impedance state and with a positive bias, the microelectrode is in a low impedance state. Under long-term soak conditions, the microelectrodes potential drifts to the negative region and a positive bias potential must be applied to obtain optimum stimulating and recording properties of the microelectrode. The electrical properties of microelectrodes measured at specific bias potentials have remained stable for many months. Two new multicontact passive semiconductor microelectrodes designs have been supplied by the University of Michigan to help determine the optimum design for size and spacing of recording sites. The microelectrodes will be evaluated in rat cortex. The results will be incorporated in future microelectrode designs.